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authorEric Blake <eblake@redhat.com>2017-11-08 15:56:58 -0600
committerEric Blake <eblake@redhat.com>2017-11-09 10:10:17 -0600
commit1104d83c726d2b20f9cec7b99ab3570a2fdbd46d (patch)
treedb4d3801912ff80f01f23d158c346060a25e255b /nbd/client.c
parente659fb3b9904b89f817e5f1d158f247be1bcc862 (diff)
nbd-client: Refuse read-only client with BDRV_O_RDWR
The NBD spec says that clients should not try to write/trim to an export advertised as read-only by the server. But we failed to check that, and would allow the block layer to use NBD with BDRV_O_RDWR even when the server is read-only, which meant we were depending on the server sending a proper EPERM failure for various commands, and also exposes a leaky abstraction: using qemu-io in read-write mode would succeed on 'w -z 0 0' because of local short-circuiting logic, but 'w 0 0' would send a request over the wire (where it then depends on the server, and fails at least for qemu-nbd but might pass for other NBD implementations). With this patch, a client MUST request read-only mode to access a server that is doing a read-only export, or else it will get a message like: can't open device nbd://localhost:10809/foo: request for write access conflicts with read-only export It is no longer possible to even attempt writes over the wire (including the corner case of 0-length writes), because the block layer enforces the explicit read-only request; this matches the behavior of qcow2 when backed by a read-only POSIX file. Fix several iotests to comply with the new behavior (since qemu-nbd of an internal snapshot, as well as nbd-server-add over QMP, default to a read-only export, we must tell blockdev-add/qemu-io to set up a read-only client). CC: qemu-stable@nongnu.org Signed-off-by: Eric Blake <eblake@redhat.com> Message-Id: <20171108215703.9295-3-eblake@redhat.com> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Diffstat (limited to 'nbd/client.c')
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